1. Field
Embodiments of the present invention relate to a solenoid valve for a brake system which improves an assembly structure of components configuring the valve to enhance durability and control performance of the valve.
2. Description of the Related Art
In general, a hydraulic brake of a vehicle performs braking operation by applying hydraulic pressure to a master cylinder through operation of a brake pedal. Here, if braking force applied to the tires exceeds static frictional force between a road surface and the tires, tire slippage occurs on the road surface.
However, a coefficient of kinetic friction is smaller than a coefficient of static friction, and thus in order to achieve optimal braking, such slippage needs to be prevented, and steering wheel lock causing an uncontrollable steering wheel needs to be prevented.
Therefore, an anti-lock brake system (ABS), which controls hydraulic pressure applied to a master cylinder to prevent tire slippage, has been proposed. The ABS basically includes a plurality of solenoid valves, an electronic control unit (ECU) to control the solenoid valves, an accumulator and a hydraulic pump.
These solenoid valves are classified into a Normally Open type, the valves of which are disposed upstream of the hydraulic brake and kept open at normal times, and a Normally Closed type, the valves of which are disposed downstream of the hydraulic brake and kept closed at normal times.
FIG. 1 is a sectional view illustrating a conventional solenoid valve of the Normally Closed type. Such a solenoid valve 10 is press-fit into a bore 15 of a modulator block 11 provided with fluid passages of a brake system, and includes a hollow seat housing 1 having an inlet 3 and an outlet 4 communicating with an inflow passage 13 and an outflow passage 14 of the modulator block 11 to enable a fluid to flow.
The seat housing 1 is hollowed inside to communicate with the inlet 3 and the outlet 4, and a valve seat 8 having an orifice 8a formed at an upper portion thereof is press-fitted into the seat housing 1.
Further, a cylindrical sleeve 6 is connected to the seat housing 1 at the upper side of the seat housing 1 such that an armature 5 installed at the seat housing 1 may move forward and backward, and a magnetic core 7 is connected to an open end of the sleeve 6 to close the open end of the sleeve 6 and move the armature 5 forward and backward.
The armature 5, which is formed of a magnetic material, moves forward and backward to open and close the orifice 8a of the valve seat 8 installed at the seat housing 1. To this end, the armature 5 is provided with an opening and closing part 5a extending toward the valve seat 8 through a through hole 2 of the seat housing 1.
A return spring 9 pressing the armature 5 is installed between the armature 5 and the magnetic core 7 so that the orifice 8a is closed by the armature 5 at normal times, and an exciting coil assembly (not shown) moving the armature 5 forward and backward is installed at outer sides of the sleeve 6 and the magnetic core 7.
In such a solenoid valve 10, when power is applied to the exciting coil assembly, magnetic force is formed between the magnetic core 7 and the armature 5 and the armature 5 is moved toward the magnetic core 7 by the magnetic force to open the orifice 8a of the valve seat 8. On the other hand, when power applied to the exciting coil assembly is interrupted, magnetic force is removed and the armature 5 is returned to its original position by elasticity of the return spring 9, thus closing the orifice 8a. 
When a magnetic field is generated in such a manner, the armature 5 opens the orifice 8a of the valve seat 8, moving toward the magnetic coil 7. When power is not applied to the exciting coil assembly, no magnetic field is generated and thus the armature 5 is operated by the elasticity of the return spring 9 to close the orifice 8a. 
The above-described solenoid valve 10 is configured to guide movement of the armature 5 using a space G between the armature 5 and the sleeve 6 when the armature 5 is operated. That is, the armature 5 moves under guidance of the sleeve 6.
Such a solenoid valve 10 needs to have operational durability due to frequent braking. In order to ensure operational durability, shaking of the armature 5 needs to be prevented when the armature 5 contacts the valve seat 8. In order to minimize such shaking, movement of the armature 5 needs to be stably guided in a region closed to the valve seat 8.
However, in the conventional solenoid valve 10, movement of the armature 5 is guided only by the space G between the armature 5 and the sleeve 6. That is, as shown in FIG. 1, since a gap S between the opening and closing part 5a formed at the lower end of the armature 5 and the seat housing 1 is relatively large, the armature 5 may not be stably guided and thus shaking of the armature 5 may occur.
Accordingly, the gap S between the armature 5 and the seat housing 1 is reduced to stably guide the armature 5. However, in this case, when power is applied to the exciting coil assembly, the reduced gap S between the armature 5 and the seat housing 1, through which a stream of magnetic force is generated, may greatly degrade responsiveness and control linearity of the solenoid valve 10 in variation of magnetic force with applied current. That is, as shown in FIG. 2, as magnetic force varies nonlinearly with current, responsiveness and control linearity of the solenoid valve 10 are low.
In addition, if the gap between the armature 5 and the seat housing 1 is reduced, responsiveness and control linearity of the solenoid valve 10 may be degraded due to frictional force encountered when the armature 5 slides.